1. Technical Field
This disclosure relates to current generators and more particularly to a temperature compensation generator for a current to be compensated in function of the difference between current temperature and a reference temperature.
2. Description of the Related Art
Central processing units (CPUs) for personal computers (PCs), workstations and servers have very sophisticated supply control mechanisms. Their power supplies meet high precision specifications both in stand-by conditions as well as in conditions of load transients. It is known that, in order to reduce costs of the output filter of these systems, “voltage position” techniques, called also “droop function” or “load line regulation” based on programming the output resistance of the power supply converter, are often used.
In order to prevent avoidable power dissipations and to sense the output current in a sufficiently refined and continuous manner, the parasitic conduction resistance DCR of the output inductor is used as sense resistance.
FIG. 1 depicts a simplified block diagram of a typical three-phase buck converter. The meaning of each functional block is summarized in the following table:
INTERLEAVINGOscillator that generates time outphasedOSCILLATORpulses for resetting ramp signalsRAMP1, RAMP2, . . . ,ramp signals mutually phase RAMPNshifted among themMULTIPHASE PWMgenerator of PWM signals mutually GENERATORphase shifted among themPWM1, PWM2, PWMNPWM signals mutually phase shifted among themMOS DRIVERdriving circuit of a power MOS stageVINsupply voltageL1, L2, . . . , LNoutput inductorsVOUToutput voltageCOUToutput tank capacitanceIINFO1, IINFO2, . . . ,reference currents of the single phasesIINFONCURRENT SHARINGcircuit for generating voltages CONTROLcorresponding to the desired reference currentsVBALANCE_1,voltages corresponding to the desiredVBALANCE_2, . . . ,reference currentsVBALANCE_NERROR AMPLIFIERerror amplifierREFreference voltageIDROOPcurrent proportional to the current supplied to the loadZFresistanceZFBfeedback resistance
In these multiphase systems, the output current of the buck converter is sensed in order to generate the desired load line. Moreover if one knows the current flowing through each channel one can implement a so-called current sharing between the phases of the system and equalize the current flowing throughout each phase for preventing stresses and damages to components.
The main problem in sensing the current on the conduction resistance DCR of the output coil is that its resistance depends on temperature. The temperature coefficient α of copper is about 0.39%, thus even small temperature fluctuations may generate relevant errors in sensing the delivered current.
The voltage read on the inductor, for example through TCM (Time Constant Matching) techniques, well known in literature, is as follows:VDCR1=IL·DCR25·[1+α(T−25)]and the current ISENSE read for a single channel by the device is
      I          SENSE      ⁢                          ⁢      1        =            I      L        ·                  DCR        25                    R        G              ·          [              1        +                  α          ⁡                      (                          T              -              25                        )                              ]      RG being the design resistance of the current sensing.Being
      I    INFO    =            I      L        ·                  DCR        25                    R        G            thenISENSE1=IINFO1·[1+α(T−25)].
For temperature compensating N currents, an equal number of thermistors, for example of NTC (Negative Temperature Coefficient) type, would be used. However, because NTC thermistors are relatively expensive, a single NTC sensing for the sum of the currents (IDROOP) is generally performed such to compensate an average temperature of the N phases. In order to do that without using additional pins, the thermistor is generally introduced in the compensation network, in place of or combined with the ZFB resistance, as shown in FIG. 2, that realizes the so-called droop function.
In FIG. 2, the block MODULATOR indicates generically the PWM signal generator and the drivers of the power stages, the block CURRENT AND THERMAL MONITOR sense the thermally compensated output current and the working temperature, and converts them in digital form for outputting the desired information, and the current ISENSE is the sum of the currents of all the phases: Iinfo1,. . . , IinfoN.
This cost saving expedient has many drawbacks:                compensation and thus the stability of the system depends on temperature;        should another thermally compensated temperature signal be desired for another use (for example the monitoring of the output current IMON), an additional thermistor would be used;        should a motherboard temperature measure (TM) be desired, a further additional thermistor would be used, with relevant increase of costs.        
A circuit that obviates to these drawbacks, disclosed by Intersil, contemplates the use of a single NTC. The solution is based on the mapping of the temperature characteristic of a known sensor. Once the temperature characteristic is known, the sensed current is corrected and this correction (that will depend upon the temperature) may be used for the various operations to be performed on the sensed current (droop function, current monitor and current sharing).
A drawback of this solution consists in that the characteristic of the sensor must be known and mapped on silicon in order to gather the correct temperature value.